In recent years, it has been an important issue to improve the fuel efficiency of automobiles in view of global environmental conservation. Therefore, it has been actively attempted that steel sheets which are materials for automobile bodies are increased in strength and reduced in thickness such that light-weight automobile bodies are achieved. However, the increase in strength of the steel sheets causes a reduction in ductility of the steel sheets, that is, reduction in the formability thereof. Hence, the following sheets are demanded: galvanized steel sheets having high strength, high formability, and excellent corrosion resistance.
The following sheets have been developed to cope with such a demand: multi-phase high-strength galvanized steel sheets such as DP (dual phase) steel sheets having ferrite and martensite and TRIP (transformation-induced plasticity) steel sheets based on the transformation-induced plasticity of retained austenite. For example, JP 11-279691 proposes a high-strength galvanized steel sheet having good formability. The sheet contains 0.05% to 0.15% C, 0.3% to 1.5% Si, 1.5% to 2.8% Mn, 0.03% or less P, 0.02% or less S, 0.005% to 0.5% Al, and 0.0060% or less N on a mass basis, the remainder being Fe and unavoidable impurities; satisfies the inequalities (Mn %)/(C %)≧15 and (Si %)/(C %)≧4; and has a ferrite matrix containing 3% to 20% martensite and retained austenite on a volume basis. DP and TRIP steel sheets contain soft ferrite and, therefore, have a problem that a large amount of an alloy element is necessary to achieve a large tensile strength TS of 980 MPa or more and a problem that stretch frangeability, which needs to be high for stretch flanging, is low because an increase in strength increases the difference in hardness between ferrite and a second phase.
JP 2003-193190 proposes a high-strength galvanized steel sheet excellent in stretch frangeability. That sheet contains 0.01% to 0.20% C, 1.5% or less Si, 0.01% to 3% Mn, 0.0010% to 0.1% P, 0.0010% to 0.05% S, 0.005% to 4% Al, and one or both of 0.01% to 5.0% Mo and 0.001% to 1.0% Nb on a mass basis, the remainder being Fe and unavoidable impurities, and has a microstructure containing 70% or more bainite or bainitic ferrite on an area basis.
However, the high-ductility, high-strength cold-rolled steel sheet specified in JP 2003-193190 does not have sufficient elongation. Thus, high-strength galvanized steel sheets having sufficient elongation and excellent stretch frangeability, excellent in formability have not yet been obtained.
It could therefore be helpful to provide a high-strength galvanized steel sheet having excellent mechanical properties such as a TS of 1200 MPa or more, an El of 13% or more, and a hole expansion ratio of 50% or more and to provide a method for manufacturing the same.
We conducted intensive studies on high-strength galvanized steel sheets having a TS of 1200 MPa or more, an El of 13% or more, and a hole expansion ratio of 50% or more and discovered the following:                i) It is effective to produce a microstructure which contains 0% to 10% ferrite, 0% to 10% martensite, and 60% to 95% tempered martensite on an area basis as determined by structure observation and which further contains 5% to 20% retained austenite as determined by X-ray diffractometry in addition to the adjustment of composition.        ii) Such a microstructure is obtained in such a manner that a steel sheet is heated from a temperature 50° C. lower than the Ac3 transformation point to the Ac3 transformation point at an average rate of 2° C./s or less, held at a temperature not lower than the Ac3 transformation point for 10 s or more, cooled to a temperature 100° C. to 200° C. lower than the Ms point at an average rate of 20° C./s or more, and then reheated at 300° C. to 600° C. for 1 to 600 s.        
We thus provide a high-strength galvanized steel sheet excellent in formability. The sheet contains 0.05% to 0.5% C, 0.01% to 2.5% Si, 0.5% to 3.5% Mn, 0.003% to 0.100% P, 0.02% or less S, and 0.010% to 0.5% Al on a mass basis, the remainder being Fe and unavoidable impurities, and has a microstructure which contains 0% to 10% ferrite, 0% to 10% martensite, and 60% to 95% tempered martensite on an area basis as determined by structure observation and which further contains 5% to 20% retained austenite as determined by X-ray diffractometry.
The high-strength galvanized steel sheet preferably further contains at least one selected from the group consisting of 0.005% to 2.00% Cr, 0.005% to 2.00% Mo, 0.005% to 2.00% V, 0.005% to 2.00% Ni, and 0.005% to 2.00% Cu on a mass basis. The high-strength galvanized steel sheet preferably further contains at least one selected from the group consisting of 0.01% to 0.20% Ti, 0.01% to 0.20% Nb, 0.0002% to 0.005% B, 0.001% to 0.005% Ca, and 0.001% to 0.005% of a REM on a mass basis.
The high-strength galvanized steel sheet may include an alloyed zinc coating.
The high-strength galvanized steel sheet can be manufactured by the following method: a slab containing the above components is hot-rolled and then cold-rolled into a cold-rolled steel sheet; the cold-rolled steel sheet is annealed in such a manner that the cold-rolled steel sheet is heated from a temperature 50° C. lower than the Ac3 transformation point to the Ac3 transformation point at an average rate of 2° C./s or less, soaked by holding the sheet at a temperature not lower than the Ac3 transformation point for 10 s or more, cooled to a temperature 100° C. to 200° C. lower than the Ms point at an average rate of 20° C./s or more, and then reheated at 300° C. to 600° C. for 1 to 600 s; and the resulting sheet is galvanized.
The method may include alloying a zinc coating formed by galvanizing.
The following sheet can be manufactured: a high-strength galvanized steel sheet having excellent mechanical properties such as a TS of 1200 MPa or more, an El of 13% or more, and a hole expansion ratio of 50% or more. The use of the high-strength galvanized steel sheet for automobile bodies allows automobiles to have a reduced weight and improved corrosion resistance.